Hydroentanglement or “spunlacing” is a process used for mechanically bonding a web of loose fibers to directly form a fabric. Such a class of fabric belongs to the “nonwoven” family of engineered fabrics. The underlying mechanism in hydroentanglement is the subjecting the fibers to a non-uniform pressure field created by a successive bank of high-velocity fluid streams. The impact of the fluid streams with the fibers, while the fibers are in contact with adjacent fibers, displaces and rotates the adjacent fibers, thereby causing entanglement of the fibers. During these relative displacements of the fibers, some of the fibers twist around others and/or interlock with other fibers to form a strong structure, due at least in part, to frictional forces between the interacting fibers. The resulting product is a highly compressed and uniform fabric formed from the entangled fibers. Such a hydroentangled fabric is often highly flexible, yet very strong, generally outperforming woven and knitted fabric counterparts in performance. The hydroentanglement process is thus a high-speed, low-cost alternative to other methods of producing fabrics. Hydroentanglement machines can, for example, produce fabric as fast as about 700 meters of fabric or more per minute, wherein the fabric may be between about 1 and about 6 meters wide. In operation, the hydroentanglement process depends on particular properties of coherent high-speed fluid streams produced by directing pressurized water through orifices defined in strips engaged with manifolds for dispensing water at a selected pressure through the orifices to form the fluid streams.
In conventional hydroentangling systems, a single manifold strip defines a double row of orifices of identical size for creating substantially identical fluid streams. In addition, it is typical to utilize a series of manifolds, wherein each presents a hydroentangling fluid stream driven by a higher pressure than the previous fluid stream. However, in such conventional systems, the aligned fluid streams create “jet streaks” in the nonwoven fabrics. Particularly, the last row of fluid streams create streaks in the nonwoven fabric because these fluid streams operate at the highest pressure, thus impacting the nonwoven fabric with the most force and creating ridges 300 (i.e. “jet streaks”) (see FIG. 3) in the finished fabric 110 in the spaces between the impact regions of the fluid streams. Also, because no processing elements and/or fluid streams are present after the last manifold in such conventional systems, the jet streaks created by the last set of fluid streams remain undisturbed and present in the finished nonwoven product produced by such systems.
The ridges 300 and/or jet streaks produced by conventional hydroentangling systems are undesirable in most of the applications where aesthetics and structural integrity of the produced fabric are important. For example, the ridges are clearly visible when the fabric is brought against light for example as in window treatments or in upholstery applications. However, eliminating and/or reducing jet streaks in hydroentangled fabrics has remained troublesome for manufacturers of nonwoven fabrics. One conventional method for obtaining a uniform surface on a hydroentangled fabric involves the introduction of transverse oscillations at regular intervals in the fluid stream curtain (see, for example, U.S. Pat. No. 6,105,222). This method involves oscillating the manifold in the transverse direction (perpendicular to the fabrics' processing direction (as described further herein). The oscillatory movement in such a technique is regulated by connecting the manifold to a reciprocating unit (such as a vibrator). This method requires a major capital investment as well as an additional source of energy for vibrating heavy manifolds. Furthermore, the final outcome of such a technique transforms the linear ridges or jet streaks into a “zig-zag” pattern without really eliminating and/or diminishing the height of such streaks. Another conventional method practiced in industry involves the introduction of 4-row nozzle-strips having nozzles with the same diameter in a staggered arrangement (see, for example, U.S. Pat. No. 6,571,441). This method also suffers from some technical problems: first, since the all the nozzles have identical diameters, the resulting fluid streams have the same impact energy and the jet-streaks caused by the last row of nozzles will permanently stay on the fabric; and second, such a technique increases the water consumption of the designated manifold by a factor of 4.
There are a few additional documented attempts at reducing and/or preventing jet-streaks in finished nonwoven fabrics. However, these hydroentangling systems have proven either inefficient or too expensive to be commercially viable. These include methods disclosed in: U.S. Pat. No. 6,877,196 (wherein fluid streams are disclosed with two opposite offset angles (towards the sides of the fabric) with respect to the vertical direction); U.S. Pat. No. 6,253,429 (disclosing a system where the fabric moves on a series of rotating drums, with manifolds placed at different angles with respect to the fabric); and U.S. Pat. No. 6,557,223 (disclosing moving the fabric transversely over a drum, combined with oscillating manifolds).
Thus, in light of the technical problems inherent in conventional hydroentanglement systems, there exists a need for an economical and practical system and method that reliably reduces the occurrence and/or magnitude of jet streaks in a nonwoven textile product.